Somnolence (alternatively sleepiness or drowsiness) is a state of strong desire for sleep, or sleeping for unusually long periods (compare hypersomnia). It has distinct meanings and causes. It can refer to the usual state preceding falling asleep, the condition of being in a drowsy state due to circadian rhythm disorders, or a symptom of other health problems. It can be accompanied by lethargy, weakness and lack of mental agility.
Somnolence is often viewed as a symptom rather than a disorder by itself. However, the concept of somnolence recurring at certain times for certain reasons constitutes various disorders, such as excessive daytime sleepiness, shift work sleep disorder, and others; and there are medical codes for somnolence as viewed as a disorder.
Sleepiness can be dangerous when performing tasks that require constant concentration, such as driving a vehicle. When a person is sufficiently fatigued, microsleeps may be experienced. In individuals deprived of sleep, somnolence may spontaneously dissipate for short periods of time; this phenomenon is the second wind, and results from the normal cycling of the circadian rhythm interfering with the processes the body carries out to prepare itself to rest.
The word "somnolence" is derived from the Latin "somnus" meaning "sleep".
Main article: Circadian rhythm sleep disorder
Circadian rhythm ("biological clock") disorders are a common cause of drowsiness as are a number of other conditions such as sleep apnea, insomnia and narcolepsy. The body clock disorders are classified as extrinsic (externally caused) or intrinsic. The former type is, for example, shift work sleep disorder, which affects people who work nights or rotating shifts. The intrinsic types include:
Sleepiness can also be a response to infection. Such somnolence is one of several sickness behaviors or reactions to infection that some theorize evolved to promote recovery by conserving energy while the body fights the infection using fever and other means. Other causes include:
Quantifying sleepiness requires a careful assessment. The diagnosis depends on two factors, namely chronicity and reversibility. Chronicity signifies that the patient, unlike healthy people, experiences persistent sleepiness, which does not pass. Reversibility stands for the fact that even if the individual goes to sleep, the sleepiness may not be completely gone after waking up. The problem with the assessment is that patients may only report the consequences of sleepiness: loss of energy, fatigue, weariness, difficulty remembering or concentrating, etc. It is crucial to aim for objective measures to quantify the sleepiness. A good measurement tool is the multiple sleep latency test (MSLT). It assesses the sleep onset latency during the course of one day—often from 8:00 to 16:00. An average sleep onset latency of less than 5 minutes is an indication of pathological sleepiness.
A number of diagnostic tests, including the Epworth Sleepiness Scale, are available to help ascertain the seriousness and likely causes of abnormal somnolence.